The present disclosure sets out a power semiconductor mounting apparatus. It involves wedging action to secure a power semiconductor to a heat sink. The heat sink set forth in the present disclosure incorporates at least two fins, one being defined as the base fin and the second being defined as the adjacent and coextensive spring fin. They are arranged with parallel roots along a heat sink base and are constructed with a modest angle between the two. The angle typically is in the range of about 2 to about 8 degrees. As will be defined, the optimum angle is perhaps 5 degrees.
A power semiconductor (hereinafter PSC) is a device required to switch many amperes of current. To handle this, they normally must be provided with a heat sink to dissipate the heat generated with such large currents flowing through the PSC. These devices are normally provided in industry standard packages, and references made to industry standard forms or packages identified as the TO-220, the TO-247 and the TO-264. These are representative devices. They are commonly made with a plastic shell around the semiconductor. On one face, there is normally a flat metal surface which is confined within the edges and is therefore a surface having a metal contact region for dissipation of electrically generated heat.
Good heat transfer is accomplished by making firm contact against the TO-220 or comparable packaging. It is undesirable to pinch the housing at an edge or corner. When this occurs, it will distribute the point loading stress lines in that region and may well cause mechanical failure of the device which will ultimately lead to electrical failure. It is just as undesirable to mount the transistor with the most common mounting mechanisms in use today, which include small machine bolts. They are often used to clamp down the industry standard containers so that the metal face is brought into physical contact with another metal surface for heat dissipation. Where that approach is used, careful alignment must be accomplished to assure that every part is planar in contact. Where contact is not planar, there is the risk of regional stress which will damage the PSC device. There is also the risk that even the most perfect of installations will work free over time when the machine bolt is anchored. The machine bolt typically must be held in place with some sort of lock washer or other split washer which prevents the bolt from backing out with the passage of time. That risk engenders another mode of failure. While installation might be acceptable at the start, vibration and other sources of movement may prompt unthreading. Vibration, unthreading, and difficulty of use are serious problems with bolted down transistor bodies.
The present disclosure is directed to an improved system for anchoring an industry standard PSC. It is placed in the inscribed V between the defined fins, namely, the base fin and the spring fin. They are constructed so that the PSC inserts easily into the gap between the two fins. The PSC is jammed against the fins, but in such a way that no damage arises. Moreover, the present invention contemplates the incorporation of a centralized raised region of specified geometric shape to be attached to the transistor shape. This is in addition to the transistor shape. In that sense, it departs somewhat, but ever so slightly, from the industry standard profile that otherwise is mandated by the semiconductor industry. To take advantage of the high performance characteristics of many PSC's, it is necessary that they be installed in a mode whereby heat is dissipated rapidly. If heat is permitted to build up in the region, then the chips may fail. Such failures are commonly dependent on heat removal. It is possible to underdesign the circuit so that a particular PSC assigned to the circuit is under performing or loafing. When this is done, the PSC will then perform in a desired fashion, but without the maximum performance. It is possible, alternatively, to design the circuit so that the transistor performs to the optimum. This, however, generates so much heat that heat sink construction is commonly critical. The prevailing or normal implementation is use of bolt down techniques. In other words, a finned aluminum heat sink is bolted to the transistor. While this will succeed, there are limitations to it and it is not the optimum way of heat transfer.
The present disclosure has revealed that a heat sink can be intimately contacted against the transistor in a better way. The PSC is clamped against one of the two fins defined above, the two fins being deployed at a small angle, and the transistor and the base fin make good face to face contact so that the temperature of the metal making up the transistor is reduced and heat liberated by this process is easily dissipated.
Another aspect of the present disclosure relates to the ease of installation. The old approach involved machine bolts. Typically, these required hand installation with a small screw driver, and required unusually small fingers to hold the bolt on one side and to hold the lock washer or split ring under the head of a nut to thread these parts together.
In volume, that is very difficult to accomplish. The present system avoids or seeks to avoid use of such screw driver assisted connections. Rather than that, this sets forth a system by which the industry standard PSC is mounted on a printed circuit board (PCB) so that easy mounting and dismounting can be done. More important, the incorporation of a PCB parallel to and commonly under the heat sink enables another feature of the present disclosure to be implemented. The PCB and the heat sink are positioned with respect to each other so that the PSC can be added after other components are fabricated on the board. For this, the PSC is slipped through the board and is inserted across the board at a rectangular slot which is sized to receive the PSC. This leaves the leads protruding on the opposite side of the PCB for easy connection. The body of the PSC is inserted and jammed up into the fins. By adding it after all the other components have been assembled, ease of alignment with the heat sink mechanism can be achieved and ease of alignment of the later installed PSC can be achieved. All of this is desirable from the point of view of improved fabrication quality, speed and improved protection against heat overload.
Heat sinks are necessary to cool power semiconductors (PSC hereinafter). Generally, data processing integrated circuit components handle sufficiently little power that cooling with a heat sink is ordinarily not necessary for them. Indeed, even where the integrated circuit density results in 500,000 or even more than 1,000,000 integrated circuit components in a data processing chip, the amount of heat that is liberated is sufficiently low that power dissipation is easily handled without resort to heavy off board mounted aluminum heat sinks. By contrast, a silicon control rectifier (SCR hereinafter) may be required to switch on and off very large current flows, perhaps 50 amperes or greater. While the voltage across the terminals and the current flow through the SCR or PSC is quite large, there is the risk of generating large heat loads during ordinary operations so that the heat must be dissipated through a heat sink. The heat sink is commonly constructed of aluminum and is normally provided with a number of fins so that the heat in the heat sink is radiated to atmosphere through the enlarged surface region provided by the heat sink. This is essential to avoid burning up the expensive circuit component. Even if the component were not expensive, repair is ordinarily expensive because it requires disassembly of circuit components, often enclosed in a cabinet or housing, and that requires that the circuit be taken out of operation. Such repairs are not welcome events.
A heat sink must be intimately contacted against the SCR or PSC to which it is attached. The heat sink must regularly be brought into firm and solid contact so that heat is transferred from the PSC or SCR. As the temperature is raised, the increase in temperature initiates a flow of heat all around the heat sink so that it is quickly brought to an elevated status, and then the heat is radiated to the surrounding atmosphere. More specifically, the heat that is formed at the heat source flows through and into the heat sink for emission by radiation or by convection currents, or by both into the surrounding atmosphere. That is accomplished from the heat sink in the intended fashion. The weak link in this heat transfer routine, however, is involved in the transfer of heat into or through the heat sink crossing the interface defined by the PSC or SCR. As a practical matter, that junction has to be dealt with securely or premature failure of the circuit component will occur. The circuit component is enclosed in a cast plastic housing. Conveniently, one face of the cast housing is a metal body through which the heat transfer is accomplished. Commonly, this body is secured to the heat sink with a fastener such as a bolt or other kind of permanent fastener. The bolt is part of the heat transfer pathway itself. There is the risk that the bolt will work loose. If it does, the heat transfer pathway is severely limited. The heat transfer pathway connects the heat flux from the PSC or SCR into the bolt and then into the thread connected heat sink body. If this is too tight, it may wrongly stress the circuit component, and if too loose, heat transfer may be inadequate, thereby resulting in difficulties of a different sort.
Physical handling of the encapsulated component must be done carefully. While the encapsulation makes the device significantly rugged, it cannot withstand heating and excessive stress applied to edges or corners. This will create a stress concentration, which in conjunction with thermal stress tends to make the device fail more readily. Such a failure is undesirable. It requires deft handling of the otherwise apparently rugged looking device. While rugged in appearance and made in a rugged fashion, they are nevertheless not so readily abused without circuit component failure. Accordingly, certain modes of attachment and securing the PSC must be avoided lest housing damage occur. This is especially true in the mounting problem, as for example with a bolt holding the heat sink.
The present disclosure sets out a method and apparatus for mounting of the thermally stressed PSC so that no damage will arise. Moreover, easy mounting and dismounting can be done without hand tools. Additionally, repairs such as replacement of a PSC can be done more readily in the assembly which is set forth below.
The primary benefit of this disclosed invention, however, relates to the ability of heat to be transferred into the heat sink. This is accomplished so that proper operating temperatures for the PSC and SCR can be achieved. Heat transfer is assured and does not depend on careful torquing of mounting bolts and the like.
The present apparatus is summarized briefly as a heat sink mounting arrangement which surrounds a PSC or SCR with two coextensive fins. One will be treated as the base fin and the other will be defined as the spring fin. It is made sufficiently thin that is deflects somewhat when bent. The fins are deployed at a divergent angle of something around 9 degrees, or less, but which angle is sufficient to apply a clamping force. The clamping force is evenly distributed over a region of the encapsulated and rugged industry standard shape or body. This enables the clamping force applied to the PSC or SCR to be distributed evenly across the central region. This jams the planar metal face of the PSC or SCR against the base fin. It is thicker and therefore more readily accepts all the heat transferred during routine operation.